Glycosylation, the enzymatic attachment of carbohydrates (glycans) to proteins and lipids is a co-translational and post-translational modification (PTM) that is more common than any other PTM as it applies to a majority of proteins synthesized in the rough endoplasmic reticulum (ER). Glycosylation plays a critical role in a variety of biological processes of membrane and secreted proteins. In the ER, glycosylation defines protein structure and folding and acts as a quality control mechanism that dictates the export of properly folded proteins to Golgi or targets misfolded ones for degradation. Glycan moieties may also act as ligands for cell surface receptors to mediate cell attachment or stimulate signal transduction pathways. Congenital disorders of glycosylation, also known as CDG syndromes, are a group of rare genetic diseases where tissue proteins and/or lipids carry defective glycosylation and/or lack of glycosylation. These diseases are linked to numerous enzymatic deficiencies and often times cause severe, sometimes fatal, impairments of the nervous system, muscles, intestines, and several other organ systems.
Common clinical symptoms in children with CDG include hypotonia, developmental delay, failure to thrive, hepatic dysfunction, coagulopathy, hypothyroidism, esotropia, abnormal fat pattern and inverted nipples, hypoglycemia, seizure, cerebellar hypoplasia, and stroke-like episodes in a developmentally delayed child. At an older age, in adolescence, and adulthood, the presentation may include ataxia, cognitive impairment, and absence of puberty in females, small testes in males, retinitis pigmentosa, scoliosis, joint contractures, and peripheral neuropathy.
CDG may be classified into two groups: CDG type I and CDG type II. CDG type I is characterized by defects in the initial steps of N-linked protein glycosylation, i.e., biosynthesis of dolichol pyrophosphate linked oligosaccharide (DLO), which occur in the ER, or transfer of the DLO to asparagine residues of nascent polypeptides. CDG type II involves defects in further processing (synthetic or hydrolytic) of the protein-bound glycan. Currently, twenty-two CDG type I and fourteen type II variants have been identified. One of the most common subtype of CDG is CDG-Ia (approximately 70% of all CDG cases), which is characterized by loss or reduction of phosphomannomutase 2 (PMM) activity leading to deficiency or insufficiency in intracellular N-glycosylation (Jaeken et al. J. of Inherit. Met. Disease. 2008, 31: 669-672). PMM as responsible for the conversion of mannose-6-phosphate to mannose-1-phosphate.
Although several different approaches to developing therapies for CDG have been explored, researchers continue their search for a suitable cure or a therapy for mitigating the disease itself. Existing treatments for manifestations include, for example, nutritional supplements, tube feeding, and a wide range of therapies that attempt to treat gastro-esophageal reflux, persistent vomiting, developmental delays, ocular abnormalities, and hypothyroidism. Patients also require intravenous (IV) hydration and physical therapy for stroke-like episodes. Adults with orthopedic symptoms often require wheel chairs, transfer devices, and surgical treatment for scoliosis (Sparks et al., Disorders of Glycosylation Overview. 2005 in: Pagon R A, Adam M P, Bird T D, et al., editors. GeneReviews™, Seattle (Wash.): University of Washington, Seattle; 1993-2013).
Currently, CDG-Ib is the only known CDG for which a relatively effective treatment is available, namely oral D-mannose administration. However, such therapy may not be as effective in treating CDG-Ia patients and there are currently limited treatment options for other CDG type I subtypes and CDG type II diseases. One of the reasons for a lack in established therapy for CDG-I disorders may be due to the plethora of heterogeneous clinical phenotypes presented that do not show a direct correlation to the PMM enzyme activity.
Patients suffering from a reduction in PMM activity have reduced production of mannose-1-phosphate (Man-1-P), associated with symptoms of multivisceral impairments. In order to overcome PMM production deficiency, it is important to supply downstream enzymes with the required substrate (i.e., Man-1-P). However, the delivery and maintenance of such a systemic supply of Man-1-P is problematic, as extracellular enzymes within bodily fluids degrade Man-1-P when delivered exogenously by oral or intravenous administration. Another problem with exogenously delivered Man-1-P is that its high polarity prevents Man-1-P from penetrating into the cell interior (i.e., cytosol) and thus treating the deficiency in PMM production.
Derivatives of the polar Man-1-P can be synthesized to make Man-1-P more cell-permeable (US Patent Publication No. 2009/0054353). This approach, however, is also problematic, as the cell-permeable Man-1-P derivative has been shown to be either unstable for clinical use or cytotoxic via the by-products of the Man-1-P derivative (Eklund et al., Glycobiology 2005, 15: 1084-1093; Rutschow et al. Bioorg Med Chem 2002, 10: 4043-4049; and Hardre et at, Bioorg Med Chem Lett 2007, 17: 152-155).
Other potential therapies have focused on inhibiting enzymes that catabolize mannose-6-phosphate (Man-6-P), a precursor to Man-1-P, via inhibition of phosphomannose isomerases (PMI). The approach focuses on forcing the reaction towards optimizing homeostasis, which with the use of PMI inhibitors, would have been skewed toward production of Man-6-P. These approaches, however, are ineffective as clinical treatment options due to their associated toxicity, off-target side effects, and poor selective tissue penetration.
Accordingly, unmet needs exist for improved compositions and methods for delivering carbohydrates, such as Man-1-P, to the cell interior in order to treat disorders, such as a congenital disorder of glycosylation (CDG), to subjects (including, for example, humans) in need of such treatment.